No abstract

At the Siberian chemical plant three heterogenerous uranium-graphite channel-type thermal reactors, in which natural uranium meal was used as the fuel in cylindrical fuel elements, have been removed from operation. All three reactors produced plutonium for weapons. The reactor I-1, a dedicated, flow-through type reactor, was put into operation in 1955 and shut down in 1990. The reactors I~I-2 and ADI~-3 were dual-purpose power reactors: The first one operated from 1958 to 1990 and the second one operated from 1961 to 1991. Highly pure (99.99%) nitrogen was blown over the graphite masonry of all three reactors. Special aluminum-based alloys served as the material for the technological pipes as well as the fuel-element claddings. All three reactors were removed from operation in accordance with the US--Russia Treaty on the cessation of plutonium production, though the technical state of the reactors ensured safe operation.Different variants of removing reactors from operation are described in the world literature --from prolonged conservation to disassembly of the plant together with rehabilitation of the plant site. The variants differ by the capital expenditures required to implement them and the dose loads received by the personnel.The variant employed in practice for removing industrial reactors from operation is unacceptable because it requires a large amount of money and, in addition, there are no objective reasons requiring that this work be done within limited time frames. The concept adopted reduces to the following.After the removal of the fuel and cool down, the core is in a state of long-time stability, i.e., the unactivated equipment is disassembled, the openings in the concrete shaft are sealed after the engineering inspection work is completed, and in this form the reactor is buried for 100 yr and longer. The reactor cavities are filled with a bentonite clay. The radioactive equipment and the systems located outside the concrete shaft of the reactor are disassembled after a holding period of 30-50 yr after the 6~ has decayed.The main problem is to prevent flooding of the reactor shaft with ground or other waters, as well as dissemination of the radioactive contamination. The top of the reactor is waterproofed with the aid of a newly produced coating, which provides protection from fires, explosions, shocks, and so on.From the moment the reactors stop, work on the disassembly of the clean and weakly contaminated equipment has been completed or is being performed: the control, monitoring, reactor shielding, water supply, fuel element unloading, and other systems are disassembled.The shutdown plan provides for execution of the program of comprehesive examination of industrial reactors. Some results of this work are examined in this paper.The graphite masonry of the reactors consists of separate blocks. The total mass of the graphite is 4600 tonnes. The maximum neutron fluence (E n > 0.18 MeV) accumulated by the graphite masonry over the entire operating period equals 1-1022 cm -2, which is close to the ...